For detecting the limit level of liquids or bulk goods in containers, capacitive measuring devices, vibratory systems or mechanical systems are applied. Besides these limit level detectors, which signal the reaching of a predetermined fill level, as soon as they come in contact with the medium, there are also contactlessly working field devices, using ultrasonic waves, microwaves or radioactive radiation. Fill level can then be determined e.g. on the basis of the travel time method. With the exception of the vibratory systems and the mechanical systems, the above-mentioned measuring or field devices are also suitable for the continuous measurement of fill level.
The field devices, independently of principle of measurement and process variable, are usually sealed in the direction of the process, against the process. Depending on application, it is necessary also to isolate the field device safely and reliably in the direction of a remote control location, relative to the remote control location. This sealing is especially important, when an aggressive or explosive atmosphere is located in the container. If e.g. an explosive gas mixture gets into the housing of the field device, then there is an acute danger of explosion, due to the direct contact with the electrical, or electronic, components of the field device. Moreover, a field device is usually connected via connection lines with a remote control location. If the field device is not sealed also in that direction, then it can not be excluded that the explosive, aggressive or poisonous gas mixture reaches the remote control location via the lines and causes damage there. A method for checking whether the seal facing the process is tight is to provide between the two seals an opening, e.g. a hole in the housing, through which the medium or gas mixture located in the container escapes from the housing, upon the occurrence of a leakage, and thus gives an obvious signal. This is, however, questionable in the case of dangerous and/or environmentally damaging substances. Furthermore, this is also scarcely acceptable in the case of hygienic applications—e.g. foods and pharmaceuticals.
An object of the invention is to provide a field device for use in dangerous and critical areas.
The object is achieved according to the invention by providing in the internal volume at least one intermediate medium, which is of a character such that it indicates via the opening the case where the first sealing stage no longer seals. The idea is, thus, that compromise of the first sealing stage is indicated not by the medium or by a gas present in the container and in communication with the medium, but, instead, by a special intermediate medium. The opening in the housing is, according to the state of the art, intended therefor, that the medium or a gas itself escapes out of the opening and, consequently indicates e.g. the leakage in the first seal. This indicating function is assumed, according to the invention, by the intermediate medium. This has the advantage that the medium does not escape first and, in so doing, possibly contaminate the environment or threaten the health of the operating personnel. The intermediate medium thus acts as a buffer. Additionally, an intermediate medium can prevent a contamination, when the intermediate medium acts, for example, to enclose the leaking medium.
One embodiment of the invention provides that the intermediate medium is coordinated with the medium. It must be prevented, for example, that mixing of medium, or gas mixture, and intermediate medium leads possibly to a still more harmful compound. It also has to be assured that the intermediate medium reacts upon the incursion of the medium or a gas associated with the medium and can indicate such.
Another embodiment provides that the intermediate medium is of a character such that it at least visually and/or acoustically and/or haptically and/or olfactorially shows that the first sealing stage is compromised. Thus, a maximum of sensory indication possibilities is used, either separately or in combination with one another. For instance, an odor can be produced, or a colored vapor, or a noise. An indication of taste would be more difficult to implement.